Novel monolithic, combo nonvolatile memory allowing byte, page and block write with no disturb and divided-well in the cell array using a unified cell structure and technology with a new scheme of decoder and layout

ABSTRACT

A nonvolatile memory array has a single transistor flash memory cell and a two transistor EEPROM memory cell which maybe integrated on the same substrate. The nonvolatile memory cell has a floating gate with a low coupling coefficient to permit a smaller memory cell. The floating gate placed over a tunneling insulation layer, the floating gate is aligned with edges of the source region and the drain region and having a width defined by a width of the edges of the source the drain. The floating gate and control gate have a relatively small coupling ratio of less than 50% to allow scaling of the nonvolatile memory cells. The nonvolatile memory cells are programmed with channel hot electron programming and erased with Fowler Nordheim tunneling at relatively high voltages.

RELATED PATENT APPLICATIONS

The present invention is a divisional application that claims priorityunder 35 U.S.C. §120 from U.S. patent application Ser. No. 10/351,180,filing date Jan. 4, 2003, now U.S. Pat. No. ______, issued ______; whichis related to and claims benefit of priority of the filing date of U.S.Provisional Patent Application Ser. No. 60/394,202 filed on Jul. 5, 2002and entitled “A Novel Monolithic Nonvolatile Memory Allowing Byte, Pageand Block Write With No Disturb and Divided-Well in The Cell Array UsingA Unified Cell Structure and Technology With A New Scheme of Decoder”,which is herein incorporated by reference; and which is further relatedto and claims benefit of priority of the filing date of U.S. ProvisionalPatent Application Ser. No. 60/426,614 filed on Nov. 14, 2002, entitled“A Novel Monolithic, Combo Nonvolatile Memory Allowing Byte, Page AndBlock Write With No Disturb And Divided-Well In The Cell Array Using AUnified Cell Structure And Technology With A New Scheme Of Decoder AndLayout ”, which is herein incorporated by reference; and which isfurther related to and claims benefit of priority of the filing date ofU.S. Provisional Patent Application Ser. No. 60/429,261 filed on Nov.25, 2002, entitled “A Novel Monolithic, Combo Nonvolatile MemoryAllowing Byte, Page And Block Write With No Disturb And Divided-Well InThe Cell Array Using A Unified Cell Structure And Technology With A NewScheme Of Decoder And Layout”, which is herein incorporated byreference.

U.S. patent application Ser. No. 09/852,247 to F. C. Hsu et al filed onMay 9, 2001 and assigned to the same assignee as the present invention.

U.S. patent application Ser. No. 09/891,782 to F. C. Hsu et al filed onJun. 27, 2001 and assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a non-volatile integrated circuitmemory. More particularly this invention relates to electricallyerasable programmable read only memory (EEPROM) and flash electricallyerasable programmable read only memory (flash memory).

2. Description of Related Art

The structure and application of the floating gate nonvolatile memoriesis well known in the art. The floating gate nonvolatile memory has threeclassifications the Electrically Programmable Read Only Memory (EPROM),the Electrically Erasable Programmable Read Only Memory (EEPROM), andthe flash Electrically Erasable Programmable Read Only Memory. The EPROMis programmed by electrically forcing charge to the floating gate.Ultra-violet light is employed to eliminate the electrical charges ofthe programming from the floating gate of the EPROM. The EEPROM and theflash memory are structurally similar at the individual cell, but havedifferent organizations. The EEPROM and the flash memory maybe have thecharge transferred to the floating gate for programming by either achannel hot injection of the charge or by Fowler-Nordheim Tunnelingthrough a tunneling oxide. The erasure of the EEPROM and Flash memory isgenerally by a Fowler-Nordheim Tunneling through the tunneling oxide.

A primary application of a nonvolatile memory is for permanent memory ina microprocessor or microcontroller system. Historically, the permanentprogram memory for the microprocessor was formed of classic maskprogrammable read only memory (ROM), and later as EPROM. Modificationsto the program memory required physically changing the memory. As theneed to update the program of the microprocessor became more important,byte-alterable EEPROM′ were developed to provide in-systemrewriteability of the memory. Further as the applications formicroprocessors and microcontrollers are becoming more pervasive, theneed for storage that is permanent and will not fail or disappear whenpower is removed is required. In most applications, the program is notmodified often. However, the data is changed relatively frequently. Theprogram memory can be classified as configuration, traceablity, bootprogram, or main program. The data includes information from anyexternal input to the system, e.g., application, instrument, recorder,or sensor data that is required for historical purposes or to maintaincontinuity of operation after power down or power loss. Data memory istypically frequently altered over the lifetime of the application.

The program memory is generally implemented in Flash memory. The Flashmemory has memory size per erase that is usually large and is in theunits of sector that ranges from 8 KB (64 K-bit) to 64 KB (6512 K-bit).Alternately, the data memory is implemented as EEPROM. The EEPROM usedfor a data memory must have segments that may be erased as small assingle byte (8 bits), to a size of a single page (128-byte), and even toerasure of the whole chip.

The ability of the EEPROM and the Flash memory to be reprogrammedrequires the device be able to be altered in system, with minimalhardware or software difficulty. The number of times the device must bealtered determines the endurance requirement of the device.Nonvolatility requires the device to retain data without power appliedfor the lifetime of the application. The lifetime of the applicationdetermines the data retention requirement of the device. Both of thereliability requirements of endurance and data retention have associatedfailure rates, which must be minimized. Since the flash memory isemployed as the program memory, it has the least amount of reprogrammingand therefore, must have the longest data retention and requires thelowest endurance (approximately 100,000 program/erase cycles).Conversely, the EEPROM, employed as data memory, must be able to bemodified repeatedly and therefore must have higher endurance (more than1 million program erase cycles).

In order to achieve the one million program/erase cycles and have thesingle-byte erase segment, the traditional EEPROM employs a very largecell size (approximately 100 times the minimum feature size of thetechnology).

Alternately, the flash memory can have a cell size that is significantlysmaller (approximately 10 times the minimum feature size of thetechnology).

In applications requiring high data rate change such as the data memory,as described, the nonvolatile memory requires a faster date change(program/erase) cycle. Thus the EEPROM requires a write or program speedof 1 ms. Alternately, the flash memory can tolerate a write speed thatis on the order of 100 ms.

FIGS. 1 a-1 d illustrates a floating gate memory cell of the prior art.The flash memory cell 10 is formed within a p-type substrate 2. An n⁺drain region 6 and an n⁺ source region 4 are formed within the p-typesubstrate 2.

A relatively thin gate dielectric or tunneling oxide 8 is deposited onthe surface of the p-type substrate 2. A poly-crystalline siliconfloating gate 12 is formed on the surface of the tunneling oxide 8 abovethe channel region 5 between the drain region 6 and source region 4. Aninterpoly dielectric layer 14 is placed on the floating gate 12 toseparate the floating gate 8 from a second layer of poly-crystallinesilicon that forms a control gate 16.

In most applications of an EEPROM or flash memory, the p-type substrate2 is connected to a substrate biasing voltage, which in most instancesis the ground reference potential (0V). The source region 4 is connectedto a source voltage generator through the source line terminal 22. Thecontrol gate 16 is connected through the word line terminal 20 to thecontrol gate voltage generator. And the drain region 6 is connectedthrough the contact 24 to the bit line and thus a bit line voltagegenerator.

The memory cell 10 is separated from adjacent memory cells or circuitsof an integrated circuit on a substrate by the shallow trench isolation26. The shallow trench isolation 26 provides a level isolation fromdisturbing signals from any operations of the adjacent cells.

As is well known, the coupling ratio of the control gate 16 and floatinggate 12 are critical in determining the magnitude of voltage appliedacross the tunneling oxide 8 to cause the flow of charge to or from thefloating gate 12. Thus it is desirable to maintain a relatively largecoupling ratio for the floating gate 12. To accomplish this, thefloating gate is extended over the shallow trench isolation 26 to formwhat is commonly termed “wings” 28. The “wings” 28 allow the voltagesapplied across the control gate 16 to be relatively lower and stillallow the charges to flow to and from the floating gate 12. However, the“wings” prevent the design of the memory cell 10 from achieving aminimum size.

According to conventional operation, the memory cell 10 is programmed byapplying a relatively high voltage (on the order of 10V) to the controlgate 16 through the word line 20. The drain voltage generator VD is setto a moderately high voltage (on the order of 5V), while the sourcevoltage generator VS is set to the ground reference potential (0V). Withthese voltages hot electrons will be produced in the channel 5 near thedrain region 6. These hot electrons will have sufficient energy to beaccelerated across the tunneling oxide 8 and trapped on the floatinggate 12. The trapped hot electrons will cause the threshold voltage ofthe field effect transistor (FET) that is formed by the memory cell 10to be increased by three to five volts. This change in threshold voltageby the trapped hot electrons causes the cell to be programmed from theunprogrammed state of a logical one (1) to a logical zero (0).

Conventionally, the memory cell is erased by setting the word line 20 toa relatively large negative voltage on the order of −18V. The bit line18 and the source line 22 may be disconnected to allow the drain 6 andthe source 4 to float. Alternately, the bit line 18 and the source line22 are connected such that the drain 6 and the source 4 are connected tothe ground reference voltage. Under these conditions there is a largeelectric field developed across the tunneling oxide 8 in the channelregion 5. This field causes the electrons trapped in the floating gate12 to flow to channel region 5, drain region 6 and source region 4. Theelectrons are then extracted from the floating gate 12 by theFowler-Nordheim tunneling. This change in threshold voltage by theremoval of the trapped hot electrons causes the cell to be erased(unprogrammed) state

If the memory cell is to be written with a logical one (1), the cell isnot programmed and no or little negative charges are placed on thefloating gate 12. Thus, if the cell is erased, the relatively largenegative voltage applied to the control gate 16 through the word line 20causes the memory cell 10 to become over-erased. Positive chargesactually are stored on the floating gate 12. This phenomenon causes theField Effect Transistor (FET) of the memory cell 10 to becomedepletion-mode transistor and the drain 6 and the source 4 to becomeessentially shorted. When this occurs, the memory cell 10 causes falsereading of data from a selected memory cell on a shared bit line of anarray having an over-erased memory cell. To overcome this problem, aselect gating transistor STx 30 is placed between the memory cell 10 andthe source line 22, as shown in FIG. 2 a-c. This prevents any excesscurrent through the memory cell 10 when the select gating transistor STx30 remains in the off-state.

Refer now FIGS. 2 a-2 c for further discussion of the two transistormemory cell of the prior art. The memory cell 10 is formed within ap-type well 36 that is formed in an n-type well 34 on a p-type substrate2. An n⁺ drain region 4 and an n⁺ source region 6 are formed within thep-type well 36.

A relatively thin tunneling oxide 8 is deposited on the surface of thep-type substrate 2. A poly-crystalline silicon floating gate 12 isformed on the surface of the tunneling oxide 8 above the channel region5 between the drain region 6 and source region 4. An interpolydielectric layer 14 is placed on the floating gate 12 to separate thefloating gate 12 from a second layer of poly-crystalline silicon thatforms a control gate 16.

The source 4 fundamentally is the drain of the select gating transistor30. The source 38 of the select gating transistor 30 is formedsimultaneously with the drain 6 and the source 4 of the memory cell 10.The gate 40 of the select gating transistor 30 is placed over the gateoxide 39 between the source 4 of the memory cell 10 and the source 38 ofthe select transistor 30.

When the tunneling oxide 8 is formed, a gate oxide 39 is formed in thechannel region between the source 4 of the memory cell 10 and the source38 of the select transistor 30. The gate 40 is connected to the selectcontrol line SG, which conducts a select signal to the select gatingtransistor 30 to control the impact of the over-erasure of the memorycell.

In most applications, of an EEPROM or flash memory having the twotransistor configuration, the p-type well 36 is connected to a substratebiasing voltage, which in most instances is the ground referencepotential (0V). The source region 38 of the select gating transistor 30is connected to a source voltage generator through the source lineterminal 22. The control gate 16 is connected through the word lineterminal 20 to the control gate voltage generator. The select gatingline 32 is connected to a select signal generator to provide the selectsignal to the gate 40 of the select gating transistor 30. And the drainregion 4 is connected through the contact 24 to the bit line 18 to a bitline voltage generator.

The memory cell 10 and select gating transistor are separated fromadjacent memory cells or circuits of an integrated circuit on asubstrate by the shallow trench isolation 26. The shallow trenchisolation 26 provides a level isolation from disturbing signals from anyoperations of the adjacent cells.

As is well known and described above, the floating gate is extended overthe shallow trench isolation 26 to form the “wings” 28. The “wings” 28allow the voltages applied across the control gate 16 to be relativelylower and still allow the charges to flow to and from the floating gate12. However, the “wings” prevent the design of the memory cell 10 fromachieving a minimum size.

The memory cell 10 is programmed by setting the drain 6 of the memorycell 10 must be set to a voltage or more than +15.0V. The control gate16 is set at the ground reference voltage level and the source 4 is madeto float by disconnecting the source line to avoid a current leakage.The +15.0V voltage present at the drain 6 and the channel 5 is coupledfrom bit line 18. The gate 40 through the select gate 32 is set to aground reference voltage. This causes the high voltage of the drain 6and the channel 5 causes a Fowler-Nordheim Tunneling of the charges fromthe floating gate 12 to drain 6.

Conventionally, the memory cell is erased by setting the word line andthus the control gate 16 are biased at around +15.0V-+17.0V. The drain 6through the bit line 18 and source 4 through the select gatingtransistor 30 and the source line 22 are both held at the groundreference voltage level. The gate 40 of the select gating transistor 30is placed at a voltage of from +3.0V-+5V and the bit line 18 is placedat the ground reference voltage to ensure that the drain 4 is set to theground reference voltage.

Other configurations of the memory cells are known in the art thatincrease the coupling coefficient through increasing the area at whichthe control gate and the floating gate are tightly coupled. Otherconfigurations effectively merge the select gating transistor and thememory cell to help improve the cell size. Still other configurationsprovide more gating transistors that isolate the memory cell from thebit line as well as the source line to prevent disturbances fromoperations on cells connected to the bit lines and the source lines.Examples of these and other configurations are described hereinafter.

U.S. Pat. No. 6,370,081 (Sakui, et al.) describes a nonvolatile memorycell having a memory cell and two select transistors sandwiching thememory cell. One block of memory nonvolatile memory cells has onecontrol gate line. The nonvolatile memory cells are connected to onecontrol gate line form one page. A sense amplifier having a latchfunction is connected to a bit line. In a data change operation, data ofmemory cells of one page are read to the sense amplifiers. After thedata is sensed and stored in the sense amplifiers a page erase isperformed. The data from the sense amplifiers are programmed in thememory cells of one page. Data in the sense amplifiers maybe changed inthe sense amplifiers prior to the reprogramming to allow byte or pagedata programming.

U.S. Pat. No. 6,400,604 (Noda) teaches a nonvolatile semiconductormemory device having a data reprogram mode. The memory has a memory cellarray, a page buffer for storing one page data to be programmed tomemory cells, which are selected in accordance with a page addresssignal. The memory further has an internal column address generatingcircuit for generating column addresses of the one page with inputtingthe page address signal in order to transfer the one data stored in thepage buffer to the memory cells, a column decoder receiving the columnaddresses from the internal column address generating circuit, and acontrol circuit having a data reprogram mode. The data reprogram modeerases one page data stored in the memory cells which are selected inaccordance with the page address signal and programs the one page datastored in the page buffer to the memory cells which are selected.

U.S. Pat. No. 6,307,781 (Shum) provides a two transistor cell NORarchitecture flash memory. The floating gate transistor is placedbetween the selection transistor and an associated bit line. The flashmemory is deposited within a triple well and operates according to aFowler-Nordheim tunnel mechanism. Programming of memory cells involvestunneling of carriers through gate oxide from a channel region to afloating gate rather than tunneling from a drain or source region to thefloating gate.

U.S. Pat. No. 6,212,102 (Georgakos, et al.) illustrates an EEPROM withtwo-transistor memory cells with source-side selection. The voltagerequired to program a memory cell is delivered via a source line.

U.S. Pat. No. 6,266,274 (Pockrandt, et al.) regards a non-volatiletwo-transistor memory cell which has an N-channel selection transistorand an N-channel memory transistor. The drive circuitry for the cellincludes a P-channel transfer transistor. A transfer channel isconnected to a row line leading to the memory cell.

U.S. Pat. No. 6,174,759 (Verhaar, et al.) teaches an EEPROM cell that isprovided with such a high-voltage transistor as a selection transistorsimilar to that described in FIGS. 2 a-c. Apart from the n-wellimplantation, high-voltage transistors of the p-channel are largelymanufactured by means of the same process steps as the p-channeltransistors in the logic, so that the number of process steps remainslimited.

U.S. Pat. No. 6,326,661 (Dormans, et al.) describes a floating gatememory cell having a large capacitive coupling between the control gateand the floating gate. The control gate is capacitively coupled to thesubstantially flat surface portion of the floating gate and to at leastthe side-wall portions of the floating gate facing the source and thedrain, and ends above the substantially flat surface portion of theselect gate. This provides a semiconductor device having a largecapacitive coupling between the control gate and the floating gate ofthe memory cell thus increasing the coupling ratio.

U.S. Pat. No. 5,748,538 (Lee, et al.), assigned to the same assignee asthe present invention, describes an OR-plane memory cell array for flashmemory with bit-based write capability. The memory cell array of a flashelectrically erasable programmable read only memory (EEPROM) includesnonvolatile memory cells arranged in rows and columns. The sources ofnonvolatile memory cells in the same memory block are connected to amain source line through a control gate. Similarly, the drains of thenonvolatile memory cells of the same memory block are connected to amain bit line. The separate source and drains in the column directionare designed for a bit-based write capability. Writing, such as erasingor programming, of a selected nonvolatile memory cell uses theFowler-Nordheim tunneling method and can be accomplished due to theprogramming or erase inhibit voltage that is applied to non-selectednonvolatile memory cells.

SUMMARY OF THE INVENTION

An object of this invention is to provide a nonvolatile memory arrayhaving a single transistor memory cell for incorporation as a Flashmemory and a two transistor memory cell for incorporation as an EEPROM.

Another object of this invention is to provide a one-transistor Flashnonvolatile memory cell having a floating gate with a low couplingcoefficient to permit a smaller memory cell.

A further objective of this invention is to provide a two-transistorEEPROM nonvolatile memory cell having a floating gate with a lowcoupling coefficient connected in series with a small select transistorto permit a smaller memory cell.

Further another object of this invention is to provide a memory array inwhich the Flash memory and EEPROM memory cells maybe integrated on thesame substrate by using the same process technology.

To accomplish at least one of these object and other objects, anonvolatile memory array is formed on a substrate. The nonvolatilememory array has nonvolatile memory cells arranged in rows and columns.Each nonvolatile memory cell has a source region and a drain regionplaced within a surface of the substrate. The drain region is placed ata distance from the source region to create a channel region within thesubstrate. A tunneling insulation layer is placed on the surface in thechannel region between the source region and drain region. A floatinggate placed over the tunneling insulation layer, the floating gate isaligned with an edge of the source region and an edge of the drainregion and having a width defined by a width of the edge of the sourceand the edge of the drain. A control gate is place over the floatinggate and isolated from the floating gate by an interlayer insulator. Thefloating gate and control gate have a relatively small coupling ratio ofless than 50% without rings to allow scaling of the nonvolatile memorycells.

Each column of nonvolatile memory had a bit line in communication withthe drain region of all nonvolatile memory cells on the column ofnonvolatile memory cells. Similarly, each row of the nonvolatile memorycells has a source line connected to the source region of allnonvolatile memory cells of the row of nonvolatile memory cells. Thenonvolatile memory array has a word line connected to the control gateof all nonvolatile memory cells of each row of the nonvolatile memorycells.

In the instance where the nonvolatile memory array has single transistornonvolatile memory cells, a selected nonvolatile memory cell isprogrammed to place a charge upon the floating gate of the selectednonvolatile memory cell by first applying a moderately high positivevoltage of from approximately +10.0V to approximately +12.0V to the wordline connected to the control gate of the selected nonvolatile memorycells. An intermediate positive voltage of approximately 5.0V is appliedto the bit line in communication with the drain region of the selectednonvolatile memory cell such that the intermediate positive voltage istransferred to the drain region and a ground reference voltage isapplied to the source line connected to the source of the selectednonvolatile memory cell.

The duration of program time for one-transistor Flash cell applying themoderately high positive gate voltage, the intermediate positive drainvoltage, and applying the ground reference source voltage is fromapproximately 1 μs to approximately 100 μs.

A selected memory cell having a single transistor is erased to removeelectrical negative charge from the floating gate by the applying a verylarge negative voltage of from approximately −15V to approximately −22Vto the word line connected to the control gate of the selected memorycell.

The source line connected to the source region of the selected memorycell and bit line in communication with the drain region of the selectednonvolatile memory cell is disconnected to allow the source region andthe drain region to float. Alternately, a ground reference voltage isapplied to the source line connected to the source region of theselected memory cell and bit line in communication with the drain regionof the selected nonvolatile memory cell during the erasing of theselected nonvolatile memory cell. The erasing of the nonvolatile memorycell has a duration of from approximately 1 ms to approximately 1 s.

The nonvolatile one-transistor Flash memory array may have nonvolatilememory cells having a gating transistor for divided bitline arrayarchitecture. The gating transistor has a source connected to the drainregion of the transistor having the floating gate by first metal. Thegating transistor also has a drain connected to the global bit line bysecond metal and a gate connected to a select line to receive selectgate signal to selectively apply a bit line voltage signal to the drainregion. The nonvolatile memory array further has a gating select lines.Each gating select line is connected to the gate of the gatingtransistor of each nonvolatile memory cell of one row of nonvolatilememory cells.

Programming to place a charge upon the floating gate of nonvolatilememory cell begins by applying a moderately high positive voltage offrom approximately +10.0V to approximately +12.0V to the word lineconnected to the control gate of the selected nonvolatile memory cells.An intermediate positive voltage of approximately 6.0V is applied to theglobal bit line connected to the drain region gating transistor of theselected nonvolatile memory cell such that the intermediate positivevoltage of 5V is transferred to the drain region of the transistor withthe floating gate. A ground reference voltage is applied the source lineconnected to the source of the transistor with the floating gate of theselected nonvolatile memory cell. A very large positive voltage isapplied to the select line connected to the gate of the gatingtransistor of the selected nonvolatile memory cell.

The duration of applying the very large positive Select-gate voltage,the moderately high positive control-gate voltage, the intermediatepositive bitline voltage, and applying the ground reference voltage toprogram the selected nonvolatile memory cell is from approximately 1 μsto approximately 100 μs.

Erasing to remove electrical charge from the floating gate of theselected nonvolatile memory cell begins with applying a very highpositive voltage of from approximately +15V to approximately +22V to theword line connected to the control gate of the gating transistor of theselected nonvolatile memory cell. The select signal is set to groundreference voltage and applied to the select line connected to the gateof the gating transistor of the selected nonvolatile memory cell. Thesource line connected to the source region of the transistor having thefloating gate of the selected nonvolatile memory cell and bit lineconnected to the drain of the gating transistor of the selectednonvolatile memory cell.

Alternately, a ground reference voltage is applied source line connectedto the source region of the transistor having the floating gate of theselected nonvolatile memory cell and bit line connected to the drain ofthe gating transistor of the selected nonvolatile memory cell. Theduration of the erasing of the memory cell is of from approximately 1 msto approximately 1 s.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d are diagrams and cross sectional views of a one transistornonvolatile floating gate memory cell of the prior art.

FIGS. 2 a-2 c are diagrams and cross sectional views of a two transistornonvolatile floating gate memory cell of the prior art.

FIGS. 3 a-3 d are diagrams and cross sectional views of a one transistornonvolatile floating gate memory cell of this invention.

FIG. 4 is schematic diagram of an array of one transistor nonvolatilememory cells of this invention.

FIGS. 5 a-5 c are diagrams and cross sectional views of a two transistornonvolatile floating gate memory cell of this invention.

FIG. 6 is a schematic diagram of an array of two transistor nonvolatilememory cells of this invention.

FIG. 7 is a plot of channel width versus select gating signal voltagefor the select gating transistor of a two transistor nonvolatile memorycell of this invention.

FIG. 8 a is a table outlining the voltage levels for programming anderasing a flash memory nonvolatile memory cell of this invention.

FIG. 8 b is a table outlining the voltage levels for programming anderasing the nonvolatile memory cell of this invention.

FIGS. 9 a and 9 b are plots showing the distribution of the thresholdvoltages of nonvolatile memory cells of this invention for programmingand erasure.

FIG. 10 is a plot of threshold voltage versus time for nonvolatilememory cell of this invention for determining duration of the programand erase operation for the nonvolatile memory cell of this invention.

FIGS. 11 a-11 m are cross sectional diagrams of a substrate illustratingthe steps for the formation of the one transistor nonvolatile memorycell of this invention.

FIGS. 12 a-12 c are cross sectional diagrams of a substrate illustratingthe additional steps for the formation of the two transistor nonvolatilememory cell of this invention.

DETAILED DESCRIPTION OF THE INVENTION

As described above the EEPROM nonvolatile memory is distinguished fromthe Flash memory in that the byte-alterable feature of the EEPROM isused to store the data code as opposed to the block (page or full chip)alterability of the Flash memory for storing program code. Since EEPROMis used for retaining information that is changed more often such asdata, the EEPROM is subjected to more programming and erasing cycles inunits of bytes and therefore, the EEPROM must have a higher endurance.In order to achieve high endurance of 1 million cycles, high-voltagebitline (program) and wordline (erase) disturbs to the non-selectedbytes have to be eliminated during repeat program and erase operations.This has led to a two transistor EEPROM nonvolatile memory cell that islarge and non-scalable but offers no bitline program disturb. Inaddition, the wordline of EEPROM cell array has been traditionallydivided to avoid wordline erase disturb to the unselected bytes.Alternately, the Flash memory is a single transistor nonvolatile memorycell having a smaller cell but requiring longer program and erase timein unit of block. To reduce the cell size of the cell and provide for aunified flash memory and EEPROM design, the present invention provides atransistor within the nonvolatile memory cell that has a floating gatethat is placed between and aligned with the edges of the source regionand drain region with no overlap. Further, the “wings” as shown in FIGS.1 a-1 d and 2 a-2 c are eliminated to decrease the coupling coefficientof the control gate and the floating gate. The decreased couplingcoefficient requires a higher control gate voltage to maintain the sameefficiency of the program and erase operations.

FIGS. 3 a-d illustrate a one-transistor floating gate flash memory cellof this invention. The nonvolatile memory cell 100 is formed within ap-type substrate 102. An n⁺ drain region 104 and an n⁺ source region 106is formed within the p-type substrate 102.

A relatively thin gate dielectric or tunneling oxide 108 is deposited onthe surface of the p-type substrate 102. A poly-crystalline siliconfloating gate 112 is formed on the surface of the tunneling oxide 108above the channel region 105 between the drain region 104 and sourceregion 106. An interpoly dielectric layer 114 is placed on the floatinggate 112 to separate the floating gate 112 from a second layer ofpoly-crystalline silicon that forms a control gate 116.

The floating gate 112 is constrained to be aligned with the edges 110 ofthe drain 104 and the source 106 over the channel region 105. Further,there are no “wings” 28 as shown in FIG. id and the floating gate isconstrained to the width of the 128 of the drain 104 and the source 106.The coupling coefficient is thus less (<50%) than the nonvolatile memorycell of FIGS. 1 a-1 d.

In an application, of a single transistor nonvolatile memory cell ofthis invention within a flash memory, the p-type substrate 102 isconnected to a substrate biasing voltage, which in most instances is theground reference potential (0V). The source region 106 is connected to asource voltage generator through the source line terminal SL 122. Thecontrol gate 116 is connected through the word line terminal WL 120 to acontrol gate voltage generator. And the drain region 104 is connectedthrough the contact 124 to the bit line 118 to a bit line voltagegenerator.

The memory cell 100 is separated from adjacent memory cells or circuitsof an integrated circuit on a substrate by the shallow trench isolation126. The shallow trench isolation 126 provides a level isolation fromdisturbing signals from any operations of the adjacent cells.

To compensate for the lower the coupling ratio of the control gate 116and floating gate 112, the magnitude of voltage applied to control gatehas to be increased to maintain the same tunneling electrical fieldacross the same thickness of tunneling oxide 108 to cause the flow ofcharge to or from the floating gate 112. In the single transistor flashnonvolatile memory cell of this invention, the voltage for theprogramming and erasure are now only few volts larger than those of theprior art using EPROM with Tunnel Oxide (ETOX) flash technology. ETOXbeing a registered trademark of Intel Corporation.

According to the operation of the nonvolatile memory cell of thisinvention as shown in FIG. 8 a, the memory cell 100 is programmed byapplying a relatively high voltage (on the order of +10.0V-+12.0V) tothe control gate 116 through the word line WL 120. The drain voltagegenerator is set to a moderately high voltage (on the order of 5V) toset the bit line BL 118 and thus the drain 104 to the moderately highvoltage, while the source voltage generator is set to the groundreference potential (0V) to set the source line SL 122 and the thus thesource 106 to the ground reference potential. With these voltages hotelectrons will be produced in the channel 105 near the drain region 104.These hot electrons will have sufficient energy to be accelerated acrossthe tunneling oxide 108 and trapped on the floating gate 112. Thetrapped hot electrons will cause the threshold voltage of the fieldeffect transistor (FET) that is formed by the memory cell 100 to beincreased by three to five volts. This change in threshold voltage bythe trapped hot electrons causes the cell to be programmed from theunprogrammed state of a logical one (1) to a logical zero (0).

The single transistor flash memory cell of this invention is erased bysetting word line generator and thus control gate 116 through the wordline WL 120 to a relatively large negative voltage of from −15.0-−22.0V,preferably −18.0V.

The bit line voltage generator and thus the bit line BL 118 and thesource line generator and thus the source SL 122 maybe disconnected toallow the drain 104 and the source 106 to float. Alternately, the bitline voltage generator and thus the bit line BL 118 and the source linegenerator and thus the source SL 122 is connected such that the drain104 and the source 106 are connected to the ground reference voltage.Under these conditions there is a large electric field developed acrossthe tunneling oxide 108 in the channel region 105. This field causes theelectrons trapped in the floating gate 112 to flow to channel region 105by the Fowler-Nordheim tunneling.

FIG. 4 illustrates an application of a single transistor flashnonvolatile memory cell of FIGS. 3 a-3 d in a block of flash memoryarray. Groups of single transistor nonvolatile memory cells 100 arearranged in rows and columns. In the flash memory, the memory cells maybe a single group having the common p-type substrate as shown in FIGS. 3a-3 d. However, the structure maybe organized as in what is commonlyreferred to as a triple-well structure, in which a large n-type well isformed on the p-type substrate and smaller p-type wells are placedwithin the n-type well. Then relatively large blocks or subarrays 200and 205 of the nonvolatile memory cells 100 are formed within theseparate p-type wells. The control gate of each nonvolatile memory cell100 of each row of the array is connected to one word line 225 a, . . ., 225 k. Similarly, the source of each nonvolatile memory cell 100 ofeach row of the array is connected to one source line 230 a, . . . , 230k. The drain of each nonvolatile memory cell 100 of each column of thearray is connected to one bit line of first metal 255 a, . . . , 255 m,260 a, . . . , 260 m.

The subarrays may in fact have vertical subarrays (not shown). Tofurther segment the array and control of the array, the individual bitlines 255 a, . . . , 255 m, 260 a, . . . , 260 m are connected to masterbit lines 245 a, . . . , 245 m, 250 a, . . . , 250 m through the gatingtransistors 210 a, . . . , 210 m, 215 a, . . . , 215 m. The drain of thegating transistors 210 a, . . . , 210 m, 215 a, . . . , 215 m of eachcolumn of the array is connected to one of the master bit lines ofsecond metals 245 a, . . . , 245 m, 250 a, . . . , 250 m. The source ofeach of the gating transistors 210 a, . . . , 210 m, 215 a, . . . , 215m is connected to drains of each nonvolatile memory cell 100 on a columnof the array. The gates of the gating transistors 210 a, . . . , 210 m,215 a, . . . , 215 m for a block 200 or 205 or for multiple blocks 200and 205 the select gating line SG 220. The source lines 230 a, . . . ,230 k for each block 200 and 205 are connected respectively to themaster source lines 240 a and 240 m respectively.

Referring back to FIG. 8 a, the programming of selected cells is asdescribed for an individual cell. The word line 225 a, . . . , 225 kcontaining the cells to be programmed (multiple cells per word line maybe programmed) is activated to the relatively high voltage (on the orderof +10.0V-+12.0V). The gate line SG 220 and thus the gate of the gatingtransistors 210 a, . . . , 210 m, 215 a, . . . , 215 m are set to veryhigh voltage (on the order of +18.0V-+22.0V) to activate the gatingtransistors 210 a, . . . , 210 m, 215 a, . . . , 215 m as opposed toonly about 10V used in prior art. With this very high voltage on thegate line SG 220, the transistors of 210 a, . . . , 210 m, 215 a, . . ., 215 m can be made much smaller than the similar devices in prior artto save silicon area. The master bit lines of second metal of 245 a, . .. , 245 m, 250 a, . . . , 250 m of the column containing the selectednonvolatile memory cells 100 is set to a moderately high voltage (on theorder of 5V). The master source lines 240 a, . . . , 240 m is set to theground reference voltage (0V). As described above this causes a channelhot electron charging of the floating gate of the selected nonvolatilememory cells 100.

The master bit lines 245 a, . . . , 245 m, 250 a, . . . , 250 m notcontaining the selected nonvolatile memory cells that are not programmed(these cells are set to a logical one (1) by the erasure) are set to theground reference potential. The gate line 220 SG is set to the very highvoltage to activate the gating transistors 210 a, . . . , 210 m, 215 a,. . . , 215 m for even those bit lines 255 a, . . . , 255 m, 260 a, . .. , 260 m not containing the selected nonvolatile memory cells duringprogram operation. Thus the drain of the nonselected memory cells not onthe bit lines 255 a, . . . , 255 m, 260 a, . . . , 260 m not containingthe selected nonvolatile memory cells are set to the ground referencepotential (0V).

The nonselected nonvolatile memory cells on the word line 225 a, . . . ,225 k containing the selected nonvolatile memory cells have theircontrol gates set to the relatively high voltage (+10.0V-+12.0V) duringprogram operation. The source of each of the nonvolatile memory cells ofthe block either selected or nonselected are set to the ground referencepotential (0V).

The nonselected nonvolatile memory bit on the bit line 255 a, . . . ,255 m, 260 a, . . . , 260 m containing the selected nonvolatile havetheir drains set to the relatively high voltage of approximately +5.0V.Since the drains and the source of the transistor having the floatinggate are set to the ground reference potential (0V), the channel hotelectron phenomena is prevented from occurring and thus disturbing thenonselected nonvolatile memory cells.

Those nonvolatile memory cells not in the same block or subarray ofnonvolatile memory cells being selected have their gate line 220, wordlines 225 a, . . . , 225 k, bit lines 255 a, . . . , 255 m, 260 a, . . ., 260 m, and source lines 230 a, . . . , 230 k set to the groundreference potential (0V) to prevent any disturbing signals within thesubarrays.

The erasure occurs for an entire block or subarray and is essentially asdescribed for an individual cell. All the word lines 225 a, . . . , 225k within the subarray are set to the very large negative voltage (on theorder of −18.0V-22.0V). The gate line 220 SG and thus the gate of thegating transistors 210 a, . . . , 210 m, 215 a, . . . , 215 m are set tothe ground reference potential (0V) to deactivate the gating transistors210 a, . . . , 210 m, 215 a, . . . , 215 m. The master bit lines 245 a,. . . , 245 m, 250 a, . . . , 250 m of the subarray are set to theground reference voltage (0V). The master source lines 240 a, . . . ,240 m and thus the source lines 230 a, . . . , 230 k of the subarray areset to the ground reference voltage (0V). As described above this causesa Fowler-Nordheim tunneling of charges from the floating gate to removeall charge from the floating gates of the nonvolatile memory cells 100of the subarray 200 and 205.

Since all the nonvolatile memory cells 100 of the subarray 200 and 205are to be erased there are no nonselected nonvolatile memory cellswithin the subarray 200 and 205. Those nonvolatile memory cells not inthe same block or subarray of nonvolatile memory cells being selectedhave their gate line 220, word lines 225 a, . . . , 225 k, bit lines 255a, . . . , 255 m, 260 a, . . . , 260 m, and bit lines 255 a, . . . , 255m, 260 a, . . . , 260 m set to the ground reference potential (0V) toprevent any disturbing signals within the subarrays.

In applications having smaller increments of erasure and requiring moreendurance (ability to withstand a high number of program and erasecycles) such as those for which the EEPROM is most applicable, the twotransistor cell is most suited to prevent the nonvolatile memory cellsfrom being over-erased. Refer to FIGS. 5 a-5 c for a description of thetwo transistor memory cell of this invention.

The memory cell 100 is formed on a p-type substrate 102. An n⁺ drainregion 104 and an n⁺ source region 106 is formed within the p-typesubstrate 102.

A relatively thin tunneling oxide 108 is deposited on the surface of thep-type substrate 102. A poly-crystalline silicon floating gate 112 isformed on the surface of the tunneling oxide 108 above the channelregion 105 between the drain region 104 and source region 106. Aninterpoly dielectric layer 114 is placed on the floating gate 112 toseparate the floating gate 112 from a second layer of poly-crystallinesilicon that forms a control gate 116.

The drain 104 fundamentally is the source of the select gatingtransistor 130. The drain 138 of the select gating transistor 130 isthrough the contact 124 to the bit line 118. The gate 140 of the selectgating transistor 130 is placed over the gate oxide 139 between thesource 108 of the memory cell 100 and the source 138 of the selecttransistor 130. The oxide 139 of gating device is thicker than tunneloxide 108 of floating-gate device 100 to withstand +18V on gatingdevices' gates during program operation.

When the tunneling oxide 108 is formed, a thicker gate oxide 139 isformed in the channel region between the drain 104 of the memory cell100 and the drain 138 of the select transistor 130. The gate 140 isconnected to the select control line 132, which conducts a select signalto the select gating transistor 130 to control the impact of theover-erasure of the memory cell.

In most application of an EEPROM or flash memory having the twotransistor configuration, the p-type substrate 102 is connected to asubstrate biasing voltage, which in most instances is the groundreference potential (0V).

The drain region 138 of the select gating transistor 130 is connected toa bit line voltage generator 6V through the contact 124 and the bit lineterminal 118. The control gate 116 is connected through the word lineterminal 120 to the control gate voltage generator. The select gatingline 132 is connected to a select signal generator to provide the selectsignal to the gate 140 of the select gating transistor 130. And thesource region 106 is connected to the source line 122 to a source linevoltage generator.

As is well known and further described above, the floating gate isextended over the shallow trench isolation 126 to form the “wings” 128.The “wings” 128 allow the voltages applied across the floating gate 112to be relatively lower and still allow the charges to flow to and fromthe floating gate 112. However, the “wings” prevent the design of thememory cell 100 from achieving a minimum size.

Similar to the one transistor nonvolatile memory cell of FIGS. 3 a-3 d,the floating gate 112 is constrained to be aligned with the edges 110 ofthe drain 104 and the source 106 over the channel region 105. Further,there are no “wings” 28 as shown in FIG. 1 d and the floating gate isconstrained to the width of the 128 of the drain 104 and the source 106.The coupling coefficient is thus less (<50%) than the nonvolatile memorycell of FIGS. 2 a-2 c.

The memory cell 100 is programmed as shown in FIG. 8 b by setting thevoltage at the drain 104 of the memory cell 100 to a voltage ofapproximately +5.0V. The control gate 116 is set at a relatively highpositive voltage level (approximately +10.0V-approximately +12.0V) andthe source 104 is grounded. The +5.0V voltage present at the drain 104and the channel 105 is coupled from bit line 118 via the select gatingtransistor 130. The gate 140 through the select gate 132 SG is set to avoltage of from approximately +17.0V-+22V. This causes the high voltageof the drain 104 and the channel 105 causes a Channel-Hot-Electron (CHE)programming to inject electrons into floating gate 112 from drain 106.

This two transistor EEPROM memory cell structure of this invention is ascalable structure since the select gating transistor 130 requiresapproximately +6.0V at the bit line 118 and 5V at drain 104 of cellduring CHE program operation. As a consequence, there is only about onevolt drop across Vds of the gating device 130. Such a low drain tosource voltage (Vds) requirement in gating device 130 and low voltage 6Vin bit line 18 does not force a higher junction breakdown and largerchannel length in the select gating transistor 130, As a result, a smallgating device 130 can be achieved within same pitch of flash cell 100 inwidth which is now suitable for technology below 0.13 μm.

The memory cell is erased by setting the word line and thus the controlgate 116 is biased at from −15.0V-−22.0V. The drain 104 through theselect gating transistor 130 and source 104 through the source line 122are both held at the ground reference voltage level. The gate 140 of theselect gating transistor 130 is placed at a voltage of from +3V and thebit line 118 is placed at the ground reference voltage to ensure thatthe drain 6 is set to the ground reference voltage. Alternately, the bitline 118 and the source line 122 maybe forced to be floating.

FIG. 6 illustrates an application of a two transistor nonvolatile memorycell of FIGS. 5 a-5 d in an EEPROM array. Groups of single transistornonvolatile memory cells 100 are arranged in units of cells 300 a, . . ., 300 k, 305 a, . . . , 305 k. These units generally are a byte, but arearranged in rows and columns. In the flash memory, the memory cells maybe a single group having the common p-type substrate as shown in FIGS. 3a-3 d. The units of cells 300 a, . . . , 300 k, 305 a, . . . , 305 k areconstructed in the preferred embodiment is formed in the P-substratewithout any triple well The control gate of each nonvolatile memory cell100 of each row of the array is connected to one word line 325 a, . . ., 325 k, 327 a, . . . , 327 k. Similarly, the source of each nonvolatilememory cell 100 of each row of the array is connected to one source line330 a, . . . , 330 k, 332 a, . . . , 332 k. The drain of eachnonvolatile memory cell 100 of each column of the array is connected toone bit line 345 a, . . . , 345 m, 350 a, . . . , 350 m.

Each nonvolatile memory cell 100 of each unit 300 a, . . . , 300 k, 305a, . . . , 305 k are connected to master bit lines 345 a, . . . , 345 m,350 a, . . . , 350 m through the gating transistors of the memory cells100 by the drain of the gating transistors. The gates of the gatingtransistors for each memory cell of the units 300 a, . . . , 300 k, 305a, . . . , 305 k on each row are connected to the select gating lines320 a, . . . , 320 k. The source lines 330 a, . . . , 330 k, 332 a, . .. , 332 k for each unit 300 a, . . . , 300 k, 305 a, . . . , 305 k areconnected respectively to the master source lines 340 a and 340 m.

Referring back to FIG. 8 b, the programming of selected cells is asdescribed for an individual cell. The word line 325 a, . . . , 325 k,327 a, . . . , 327 k containing the cells to be programmed (multiplecells per word line within each selected unit (byte) may be programmed)is activated to the relatively high voltage (on the order of+10.0V-+12.0V). The gate line 320 and thus the gate of the gatingtransistors memory cells 100 of the unit of cells 300 a, . . . , 300 k,305 a, . . . , 305 k to be programmed are set to very high voltage (onthe order of +18.0V-+22.0V) to activate the gating transistors gatingtransistors memory cells 100 of the unit of cells 300 a, . . . , 300 k,305 a, . . . , 305 k. The bit lines 345 a, . . . , 345 m, 350 a, . . . ,350 m of the column containing the selected nonvolatile memory cells 100are set to a moderately high voltage (on the order of 6.0V). The mastersource lines 340 a, . . . , 340 m is set to the ground reference voltage(0V). As described above this causes a channel hot electron charging ofthe floating gate of the selected nonvolatile memory cells 100.

The bit lines 345 a, . . . , 345 m, 350 a, . . . , 350 m not containingthe selected nonvolatile memory cells that are not programmed (thesecells are to be set to a logical one (1) by the erasure) are set to theground reference potential. The select gating lines 320 a, . . . , 320 kare set to the very high voltage level (+15.0V-+22.0V) to activate theselect gating lines 320 a, . . . , 320 k for even those bit lines 355 a,. . . , 355 m, 360 a, . . . , 360 m not containing the selectednonvolatile memory cells 100. Thus the drain of the nonselected memorycells not on the bit lines 355 a, . . . , 355 m, 360 a, . . . , 360 mnot containing the selected nonvolatile memory cells are set to theground reference potential (0V).

The nonselected nonvolatile memory cells on the word line 325 a, . . . ,325 k, 327 a, . . . , 327 k containing the selected nonvolatile memorycells have their control gates set to the relatively high voltage(+10.0V-+12.0V). The source of each of the nonvolatile memory cells ofthe block either selected or nonselected are set to the ground referencepotential (0V).

The nonselected nonvolatile memory bit on the bit lines 345 a, . . . ,345 m, 350 a, . . . , 350 m containing the selected nonvolatile havetheir drains set to the relatively high voltage (+6.0V). Since thedrains and the source of the transistor having the floating gate are setto the ground reference potential (0V), the channel hot electronphenomena is prevented from occurring and thus disturbing thenonselected nonvolatile memory cells 100.

Those nonvolatile memory cells not in unit of cells 300 a, . . . , 300k, 305 a, . . . , 305 k being selected do not have their select gatinglines 320 a, . . . , 320 k, word lines 325 a, . . . , 325 k, 327 a, . .. , 327 k, source lines 330 a, . . . , 330 k, 332 a, . . . , 332 k, andbit lines 355 a, . . . , 355 m, 360 a, . . . , 360 m set to the groundreference potential (0V) to prevent any disturbing signals within thenonselected units 300 a, . . . , 300 k, 305 a, . . . , 305 k.

The erasure occurs for an entire unit 300 a, . . . , 300 k, 305 a, . . ., 305 k of memory cells 100 or groups of units 300 a, . . . , 300 k, 305a, . . . , 305 k of memory cells 100 and is essentially as described foran individual cell. The word line 325 a, . . . , 325 k, 327 a, . . . ,327 k within a selected unit (or units) 300 a, . . . , 300 k, 305 a, . .. , 305 k of memory cells 100 are set to the very large negative voltage(on the order of −15.0V-−22.0V). The select gating lines 320 a, . . . ,320 k of the selected unit 300 a, . . . , 300 k, 305 a, . . . , 305 kand thus the gate of the gating transistors 310 a, . . . , 310 m, 315 a,. . . , 315 m are set to the ground reference potential (0V) todeactivate the gating transistors 310 a, . . . , 310 m, 315 a, . . . ,315 m. The bit lines 345 a, . . . , 345 m, 350 a, . . . , 350 m of theselected unit 300 a, . . . , 300 k, 305 a, . . . , 305 k are set to theground reference voltage (0V). The master source lines 340 a, . . . ,340 m and thus the source line 330 a, . . . , 330 k, 332 a, . . . , 332k of the selected unit 300 a, . . . , 300 k, 305 a, . . . , 305 k areset to the ground reference voltage (0V). As described above this causesa Fowler-Nordheim tunneling of charges from the floating gate to removeall charge from the floating gates of the nonvolatile memory cells 100of the selected unit 300 a, . . . , 300 k, 305 a, . . . , 305 k.

Since all the nonvolatile memory cells 100 of a selected unit 300 a, . .. , 300 k, 305 a, . . . , 305 k of memory cells, those nonvolatilememory cells not in the same unit 300 a, . . . , 300 k, 305 a, . . . ,305 k of nonvolatile memory cells 100 being selected have their selectgating lines 320 a, . . . , 320 k, word lines 325 a, . . . , 325 k, 327a, . . . , 327 k, and bit line 345 a, . . . , 345 m, 350 a, . . . , 350m set to the ground reference potential (0V) to prevent any disturbingsignals within the subarrays.

FIG. 7 is a plot that illustrates the relationship between the selectgating transistor's gate voltage for the gating transistors 210 a, . . ., 210 m, 215 a, . . . , 215 m of FIG. 4 or the select gating transistor130 of each two transistor nonvolatile memory cell of FIGS. 5 a-5 c. Thebit line voltage generator of a charge pump circuit of a memory arraysets the bit line to a voltage of approximately 6.5V. This is on theassumption that the memory cell requires a drain voltage 5V and draincurrent 500 μA to perform channel hot electron programming. The selectgating transistor is designed to have a channel length is fixed at 0.4μm. The plot illustrates the minimum channel width required to providethis required condition under different gate voltages. It is shown thatthe channel width must be 1.7 μm for the nonvolatile memory cells of theprior art where the control gate is set to a voltage 10V. However, forthe control gate voltage of a memory cell of this invention, the voltageapplied is increased to 20V. This allows the select gating transistor'schannel width can be drastically reduced to only 0.45 μm. This allowsthe select gating transistor to have a sufficiently small size to fitinto the memory cell's pitch (width of a column of memory cells in anarray. The two transistor nonvolatile memory cell constructed within anEEPROM array contains identical memory cell structure as the onetransistor nonvolatile memory cell constructed in Flash memory array ofthis invention. Both array structures use the identical Fowler Nordheimchannel erase and Channel Hot Electron program schemes. This allows forthe integration of EEPROM array structures and the flash memory arraystructures within the same integrated circuit on a substrate.

FIGS. 9 a and 9 b illustrates the distribution of the voltage thresholdgroups of nonvolatile memory cells of this invention. The voltage thatis used as the reference threshold voltage during a read operation(Vread) is defined as the demarcation between the erased (logical 1) andthe programmed (logical 0) for the nonvolatile memory cells. Since theamount of charge that is removed from the floating gate during erasureor placed on the floating gate during programming varies, the voltagethreshold for the nonvolatile memory cells has a distribution as shown.The bit lines containing the selected nonvolatile memory cells of anarray are set to a voltage sufficient to detect whether the selectednonvolatile memory cells is programmed or erased and the word linecontaining the selected nonvolatile memory cell is placed at thereference threshold value (Vread). If the selected nonvolatile memorycell is erased, the selected nonvolatile memory cell is turned on andthe logical 1 is detected. Alternately, if the selected nonvolatilememory cell is programmed, the selected nonvolatile memory cell is notturned on and the logical 0 is detected.

As described above, those nonvolatile memory cells that have not beenprogrammed (erased) and are then subjected to a repeated erase operationmay become over-erased as shown in FIG. 9 b. Conversely, thosenonvolatile memory cells in a block of nonvolatile memory cells having afaster erase speed may also be subject to over-erasure. The floatinggate transistor of the nonvolatile memory cell is essentially operatingin the enhancement mode and conducts (turned on) at all times. Thiscondition can not be tolerated for the one transistor Flash nonvolatilememory cell of FIGS. 3 a-3 d. However, the two transistor EEPROMnonvolatile memory cell of FIGS. 5 a-5 d tolerates the over-erased celland prevents corruption of data in other cells by preventing of the flowof current in the bit lines through the enhanced floating gatetransistor. A special operation called ‘correction, ‘repair’, ‘recover’,‘converge’, or ‘soft-programming’ may be adopted to adjust the thresholdvoltage of those over-erased nonvolatile memory cells back to becentered on the desired reference threshold voltage (Vread) for Flashcell.

Since the single transistor Flash nonvolatile memory cell and the twotransistor EEPROM nonvolatile memory cell of this invention utilize thesame structure for applications as the EEPROM and the flash memory, theamount of time for programming and erasure can be identical. Inpractice, Flash erase time is around few hundred mS, while EEPROM is fewmS in product spec. Both memory use's same scheme of CHE forprogramming, thus the program time are identical from 1 μs to 100 μsrange in today's product spec. Refer now to FIG. 10 for a discussion ofthe program and erase times for the nonvolatile memory cell of thisinvention. The amount of time to change the threshold voltage withchannel hot electron programming is shown in the plot 90. While theamount of time for removal of the charges from the floating gate withFowler Nordheim tunneling to erase the nonvolatile memory cell is shownin plot 95. The time for the application of the necessary voltages asdescribed for the programming of the nonvolatile memory cell has aduration of from approximately 1 μs to approximately 10 μs. The erasureof the nonvolatile memory cell has a duration of from approximately 1 μsto approximately 1 μs.

Refer now to FIGS. 11 a-11 m for a discussion of the method forfabrication of the nonvolatile memory cell 100 and the gatingtransistors 210 a, . . . , 210 m, 215 a, . . . , 215 m of the flashmemory structure of FIG. 4 or the gating transistor 130 of the twotransistor nonvolatile memory cell of FIGS. 5 a-5 c. In thisillustration, the floating gate transistor 100 and the select gatingtransistor 130 have a stack gate structure comprising of twopolycrystalline silicon layers (poly-1 and poly-2) but only the controlgate 116 and floating-gate 112 of cell 100 is separated by theinter-poly dielectric 114. The poly-2 control gate and poly-1 floatinggate of select gating device 130 is shorted without separation. For thefloating gate transistor 100, the poly-1 is used as the floating gate,and the poly-2 is used as the control gate. The select gate transistor130 has also a stack gate structure, but the gate voltage is directlyapplied to the poly-1 layer.

Referring FIG. 11 a, in which a p-type silicon substrate 400 with <100>crystallographic orientation is provided. An implant oxide 402 is nextformed on the silicon substrate 400 by thermal oxidation or depositionwith a thickness between about 100 Å to 300 Å. A layer of photoresist404 is then deposited and patterned by the mask of select gatetransistor 130 area. The select gate transistor 130 area is thenimplanted 406 with a threshold voltage adjustment (Vt) implant followedby a field implant. Both implants use a p-type impurity such as boron orboron difluoride (BF₂) to adjust the threshold voltage of select gatetransistor 130 and the field transistor turn-on voltage. The thresholdvoltage of the select gate transistor 130 is between about 0.6 to 1.5 V,and the threshold voltage of the field transistor is generally largerthan 18 V. The select gate transistor 130 Vt implant has energy betweenabout 5 to 50 KeV and a dose between about 3E11 to 5E12 ions/cm² usingboron ions. The field implant 406 for the select gate transistor 130 hasan implant energy between about 30 to 180 KeV and a dose between about1E12 to 1E14 ions/cm² using boron ions. The field implant has higherimplant energy than the select gate transistor 130 Vt implant becausethe implanted ions need to penetrate the field oxide. During program,the select gate voltage is between about 14 to 20 V, which is higherthan word line voltage. The field implant is therefore required toincrease the threshold voltage of the field transistor to insure thefield transistor is not turned on when select gate has a high voltage.

After the photoresist 404 and the implant oxide 402 are stripped, alayer of high voltage (HV) gate oxide 408 of a thickness between about100 to 300 Å is then thermally grown on the silicon substrate 400 asshown in FIG. 11 b. The silicon substrate 400 is then patterned by thecell transistor photolithography to form the photoresist 410. Thefloating gate transistor 100 area is implanted 412 with nonvolatilememory cell transistor 100 Vt implant and field implant. The cell Vtimplant is used to adjust the threshold voltage of the nonvolatilememory cell transistor 100, which is between about 1.0 to 3.0 V. Thefield implant is to increase the threshold voltage of the N-fieldtransistor not shown in drawing to higher than 20.0 V. The nonvolatilememory cell transistor 100 Vt implant has energy between about 5 to 50KeV and a dose between about 1E12 to 1E13 ions/cm² using boron ions. Thefield implant has energy between about 30 to 180 KeV and a dose betweenabout 1E12 to 1E14 ions/cm² using boron ions.

The HV gate oxide 408 is then removed in the nonvolatile memory celltransistor 100 area as shown in FIG. 11 d. The photoresist 414 is thenremoved. The tunnel oxide 416 of a thickness between about 70 to 120 Åis then thermally grown on the silicon wafer by the conventional dryoxidation process at a temperature between about 900 to 1100° C. asshown in FIG. 11 e. The tunnel oxide 416 grown on the select gatetransistor 130 area is thinner than the oxide grown on the nonvolatilememory cell transistor 100 area, because the HV gate oxide 408 alreadyexists on that area. The tunnel oxide 416 and HV gate oxide 408 arecombined to be the gate oxide for the select gate transistor 130, whichis between about 150 to 350 Å.

Referring to FIG. 11 f, the first polysilicon layer (Poly 1) 418 is thendeposited, using LPCVD procedures, at a thickness between about 1000 to2000 Å. The polysilicon layer 418 is then patterned by the poly-1 photo.The polysilicon layer 418 in the poly-1 window area is removed.

An inter-polysilicon dielectric layer 420, such as silicon dioxide,silicon nitride, or oxide/nitride/oxide composite layer, is nextdeposited using low pressure chemical vapor deposition (LPCVD), PlasmaEnhanced Chemical Vapor Deposition (PECVD), or high density plasmachemical vapor deposition (HDPCVD), or thermal oxidation procedures canalso be used to create the silicon oxide option, all resulting in athickness between about 100 to 300 Å as shown in FIG. 11 g. A depositionof a second polysilicon layer 422 follows, using LPCVD procedures, at athickness between about 1500 to 3000 Å, again in situ doped, duringdeposition, via the addition of arsine, phosphine, to a silane ambient,or add a layer of Tungsten Silicide (WSi) to be used subsequently forthe control gate of the nonvolatile memory cell 100 as shown in FIG. 11h.

Photolithographic and reactive ion etching (RIE) procedures are nextemployed to create stacked gate structure with the poly 1 418 and thepoly 2 422 layer, schematically shown in cross sectional representationin FIG. 11 i. The anisotropic RIE procedure is performed using chlorine(Cl₂) for second polysilicon layer 422, and for first polysilicon layer418, while a fluorine containing gas CHF₃ is used to patterninter-polysilicon dielectric layer 420. The stacked gate structure ofthe nonvolatile memory cell 100 has the control gate formed of thesecond polysilicon layer 422, inter-polysilicon dielectric layer 420,and floating gate formed of the first polysilicon layer 418. The stackedgate structure as described resides on tunnel oxide layer 416.

Referring to FIG. 11 j, a layer of photoresist 424 is deposited. Thememory cell photo mask defines the nonvolatile memory cell 100 area,from which the photoresist is removed. The nonvolatile memory cellsource 428 and drain 430 junctions are next formed via an ionimplantation 426 procedure, at an energy about 30 to 60 KeV, and at adose between about 1E15 to 7E15 ions/cm², using arsenic ions. Thenonvolatile memory cell drain 430 junction is an abrupt junction topromote impact ionization of channel hot electrons. The memory cellimplant 426 is to create a heavily doped drain (HDD) junction for thecell drain.

Further the nonvolatile memory cell source 428 and drain 430 arestructured to align with edges of the stacked gate structure of thefloating gate and the control gate of the nonvolatile memory cell 100.As noted in FIG. 3 d, the stacked gate structure is constrained withinthe shallow trench isolation (not shown) that demarcates the boundary ofthe nonvolatile memory cell.

The drain junction 436 of the select gate transistor 130, shown in FIG.11 k requires sustaining a higher junction breakdown voltage than thenonvolatile memory cell drain 430 junction. The impact ionization nearthe drain junction 430 of the select gate transistor 130, on the otherhand, is not desired. The drain junction 436 of the select gatetransistor 130 has a different doping profile from the nonvolatilememory cell drain 430 junction. The select gate transistor drain 436region is defined by the select gate transistor 130 photo mask 432, andimplanted 434 with phosphorus ions at an energy about 50 to 150 KeV, andat a dose between about 1E14 to 2E15 ions/cm². The implant 434 is tocreate a double diffused drain (DDD) junction 436 for the select gatetransistor 130 drain. The DDD junction 436 has a more gradual dopingprofile than the memory cell drain 430 junction.

Insulator spacers 438, schematically shown in FIG. 11 l, are formed viadeposition of an insulator layer, such as silicon nitride, via LPCVD orPECVD procedures, at a thickness between about 1000 to 2000 Å, followedby an anisotropic RIE procedure, using a fluorine based compound (CF₄)as an etchant. Source/drain n+ implant 440 is then implanted via an ionimplantation procedure, at an energy between about 30 to 60 KeV, at adose between about 5E14 to 1E16 ions/cm² using arsenic or phosphorusions to reduce the source/drain(442, 444, 446) series resistance.

The process is continued by depositing interlevel dielectric (ILD)layer, comprised of silicon dioxide, obtained via LPCVD or PECVDprocedures, at a thickness between about 8000 to 15,000 Å. The ILDcompletely fills the spaces between stacked gate structures.Planarization of ILD layer is then accomplished via a chemicalmechanical planarization (CMP) procedure, resulting in a smooth topsurface topology for ILD layer, reducing the severity of the subsequent,photolithographic procedure, used for the openings to the source/drainregions. The contact hole opening is created via a RIE procedure of theILD layer, using a fluorine based etchant such as CHF₃. A metal layersuch as tungsten, at a thickness between about 3000 to 4000 Å, isdeposited via LPCVD procedures, using tungsten hexafluoride as a source,or via RF sputtering procedures, completely filling the contact holeopening. Removal of the tungsten layer, residing on the top surface ofILD layer, is accomplished via a CMP procedure, or via a selective RIEprocedure, using Cl₂ as an etchant. The tungsten plug is to provide theelectric contact to the source, drain , and the polysilicon gate. Ametal layer, such as an aluminum layer is then deposited, via RFsputtering, to a thickness between about 3000 to 8000 Å to provide thefirst metal interconnects.

The stacked gate structure (418, 420, and 422) of the select gatetransistor has an external connection of the poly 1 gate to the selectgating lines 320 a, . . . , 320 k of FIG. 6. The process steps for thenonvolatile memory cell and the select gating transistor as shown may beimplemented by concurrent semiconductor process of any currentsemiconductor process and will be applicable as advances in processsteps become available in the future. The high voltage relatedreliability issue can be optimized by carefully choosing the parametersof the cell structure such as channel length, gate oxide thickness,shallow trench isolation depth, etc. The present semiconductorprocessing of a NAND-type-array Flash memory nonvolatile cells has beenproven to have very high reliability. The process is able to sustainhigher than 20V with more than 1 million cycles endurance, which issufficient to manufacture the nonvolatile memory cell of this invention.

A variation of the fabrication process is described as follows in FIGS.14 a-14 c. This variation is a self-aligned process, in which the firstpolysilicon is self-aligned to the field oxide in order to reduce thewidth of the nonvolatile cell size to the width of the source and drainof the nonvolatile memory cell 100 as shown in FIG. 3 d. The shallowtrench isolation 126 of FIG. 3 d is formed after the first polysilicon418 deposition so that the first polysilicon 418 is self-aligned to theactive edge of the nonvolatile memory cell 100 of FIG. 14 a.

The deposition and formation of the first polysilicon layer 418 can bemade by the well-known process such as Low Pressure CVD or LPCVDresulting in a layer of approximately 500-650 Å thickness of polysiliconon the tunnel oxide as described above. A silicon nitride layer ofpreferably 1500 Å is deposited by CVD. The photo mask of active area isused to define the active regions during isolation formation. A layer ofphotoresist is applied on the silicon nitride layer and a masking stepis performed to etch the silicon nitride, the first polysil, and theunderlying insulating layer in the selective regions. Where thephotoresist is not removed, they remain on top of the silicon nitride,the first polysilicon region, and the underlying insulating materialregion. Where the photoresist is removed, the silicon nitride, thepolysilicon, and the underlying insulating material are etched away.There are two methods for the formation of the isolation regions: Localoxidation of silicon (LOCOS) and shallow trench isolation. In theshallow trench isolation method, the etching continues into thesubstrate to a depth of approximately 2800 Å˜3200 Å. The silicontrenches are filled with an isolation material such as silicon dioxide.This can be the well-known LOCOS process resulting in the local fieldoxide or it can be shallow trench isolation (STI) process resulting inthe silicon dioxide being formed in the region. In the preferred method,the shallow trench will be formed. Shallow trench isolation is desirablebecause it can be formed planar with respect to the first polysiliconlayer 418. The structure the floating gate 418 of the nonvolatile memorycell 100 is self-aligned to the source and drain of the nonvolatilememory cell 100.

After the isolation region is formed, an inter-polysilicon dielectriclayer 420, such as silicon dioxide, silicon nitride, oroxide/nitride/oxide composite layer, is next deposited using LPCVD,PECVD, or high density plasma chemical vapor deposition (HDPCVD), orthermal oxidation procedures can also be used to create the siliconoxide option, all resulting in a thickness between about 100 to 300 Å asshown in FIG. 14 a. The inter-polysilicon dielectric layer 420 is thenpatterned by the photo mask of oxide-nitride-oxide (ONO). Theinter-polysilicon dielectric layer 420 is removed in the select gatetransistor 130 area as shown in FIG. 14 b by the anisotropic RIEprocedure using fluorine compounds (CHF_(x)). A second polysilicon layer422 is deposited as described in FIG. 11 h, using LPCVD procedures, at athickness between about 1500 to 3000 Å, again in situ doped, duringdeposition, via the addition of arsine, phosphine, to a silane ambient,or to add a layer of WSi to be used subsequently for the control gate ofthe nonvolatile memory cell.

Photolithographic and RIE procedures are next employed to create stackedgate structure (418, 420, and 422) schematically shown in crosssectional representation, in FIG. 14 c. The second polysilicon layer 422is deposited such that it is direct electrical contact with the firstpolysilicon layer 418 to form the gate for the select gate transistor130.

The single and two transistor nonvolatile memory cell allow for ascalable memory array that may be used for either a flash memory or foran EEPROM employing the same nonvolatile cell structure. This enablesthe combination of the flash memory and the EEPROM as discrete memory orembedded memory within an integrated circuit. The single and twotransistor nonvolatile memory cells as used in a flash memory and anEEPROM use an identical nonvolatile memory manufacture technology anderase scheme, thus having equivalent performance. The nonvolatile memorycell of this invention permits a small die size, superior endurancecycle, and high flexibility.

While this invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method for forming a nonvolatile memory cell comprising steps offorming a source region and drain region spatially separated within asurface of a substrate; forming a tunneling insulator upon said surfaceof said substrate in a channel region between the source region and thedrain region; forming a floating gate placed over the channel region ofsaid memory cell; aligning said floating gate with an edge of saidsource region and an edge of said drain region; setting a width of saidfloating gate to be a width of said edge of said source and said edge ofsaid drain; and forming an insulating layer upon said floating gate; andforming a control gate upon said insulating layer above said floatinggate.
 2. The method for forming the nonvolatile memory cell of claim 1further comprising the step of defining an area of said floating gatesuch that said nonvolatile memory cell has a relatively small couplingratio of capacitance formed by a control gate placed over said floatinggate to a total capacitance of said floating gate and said control gate.3. The method for forming the nonvolatile memory cell of claim 2 whereinsaid coupling ratio is less than 50%.
 4. The method for forming anonvolatile memory cell of claim 1 further comprising the steps of:connecting said control gate to a word line; placing said drain regionin communication with a bit line; and connecting said source region to asource line.
 5. The method for forming the nonvolatile memory cell ofclaim 2 wherein said the nonvolatile memory cell is programmed to placea charge upon said floating gate by the steps of: applying a moderatelyhigh positive voltage to said control gate through said word line;applying an intermediate positive voltage to said drain region throughsaid bit line; and applying a ground reference voltage to said sourceregion through said source line.
 6. The method for forming thenonvolatile memory cell of claim 5 wherein said moderately high positivevoltage is from approximately +10.0V to approximately +12.0V.
 7. Themethod for forming the nonvolatile memory cell of claim 5 wherein saidintermediate positive voltage is approximately 6.0V such thatapproximately 5.0V is applied to the drain region.
 8. The method forforming the nonvolatile memory cell of claim 5 wherein applying saidmoderately high positive voltage, said intermediate positive voltage,and applying said ground reference voltage has a duration of fromapproximately 1 μs to approximately 100 μs.
 9. The method for formingthe nonvolatile memory cell of claim 4 wherein said memory cell iserased to remove electrical charge from said floating gate by the stepsof: applying a very large negative voltage to said control gate throughsaid word line.
 10. The method for forming the nonvolatile memory cellof claim 9 wherein the very large negative voltage is from approximately−15V to approximately −22V.
 11. The method for forming the nonvolatilememory cell of claim 9 wherein erasing said memory cell furthercomprises the step of: disconnecting the source line and said bit lineto allow said source region and said drain region to float.
 12. Themethod for forming the nonvolatile memory cell of claim 9 whereinerasing said memory cell further comprises the step of: applying aground reference voltage to said source region through said source lineand said drain region through said bit line.
 13. The method for formingthe nonvolatile memory cell of claim 9 wherein erasing said memory cellhas a duration of from approximately 1 ms to approximately 1 s.
 14. Themethod for forming the nonvolatile memory cell of claim 4 furthercomprising the step of forming a gating transistor by the steps of:forming a source; connecting said source to the drain region, forming adrain, connecting said drain to the bit line; forming a gate; connectingsaid gate to a select line to selectively apply a bit line voltagesignal to the drain region.
 15. The method for forming the nonvolatilememory cell of claim 14 wherein said nonvolatile memory cell isprogrammed to place a charge upon said floating gate by the steps of:applying a moderately high positive voltage to said control gate throughsaid word line; applying an intermediate positive voltage to said drainregion through said gating transistor from said bit line; applying avery large positive voltage to said gate of said gating transistorthrough said gate line; and applying a ground reference voltage to saidsource region through said source line.
 16. The method for forming thenonvolatile memory cell of claim 15 wherein said moderately highpositive voltage is from approximately +10.0V to approximately +12.0V.17. The method for forming the nonvolatile memory cell of claim 15wherein said intermediate positive voltage is approximately 6.0V suchthat approximately 5.0V is applied to the drain region.
 18. The methodfor forming the nonvolatile memory cell of claim 15 wherein the verylarge positive voltage is from approximately +15V to approximately +22V.19. The method for forming the nonvolatile memory cell of claim 15wherein applying said very large positive voltage, said moderately highpositive voltage, said intermediate positive voltage, and applying saidground reference voltage has a duration of from approximately 1 μs toapproximately 100 μs.
 20. The method for forming the nonvolatile memorycell of claim 14 wherein said memory cell is erased to remove electricalcharge from said floating gate by the steps of: applying a very highpositive voltage to said control gate through said word line; andapplying a ground reference voltage to said select gate through saidselect line.
 21. The method for forming the nonvolatile memory cell ofclaim 20 wherein erasing said memory cell further comprises the step of:disconnecting the source line and said bit line to allow said sourceregion and said drain region to float.
 22. The method for forming thenonvolatile memory cell of claim 20 wherein erasing said memory cellfurther comprises the step of: applying a ground reference voltage tosaid source region through said source line and said drain regionthrough said gating transistor from said bit line.
 23. The method forforming the nonvolatile memory cell of claim 20 wherein erasing saidmemory cell has a duration of from approximately 1 ms to approximately 1s.